I'd really love to see this idea getting through.
If there was any way possible to work around the problem miles mentioned. This would be both fun and useful on so many levels. And indeed, for those who rp alot, it would be a nice addition.
I know I'd find this function handy alot of times if it was there.
If there's a way to detect the character sizes(Which I'm sure there is). It would pretty much be down to how much work in making the animations and making sure it doesn't spazz on other players screen.
Not only would it be fun to carry cubs and pups around, but I know for a fact that I have told friends to go to another place and they have been lazy, maybe if you could just let them lay on your back while you walk there it would be fun and no excuse for lazyness anymore, and it would be fun.
Ofcourse, if you would be able to do this, you shouldn't be able to jump that high, if not at all. And the running should be slower, if no running at all aswell. Maybe just some faster walking or "jogging"? If you know what I mean.
But that means alot more work aswell. And it's probably alot of work just making the animations for carrying cubs/pups and carrying someone on your back + a system that detects the sizes.
This idea is gold however. And wish for it to be added sometime in the future. It would really spice things up.